The term shape memory alloy (SMA) is applied to that group of metallic compounds that demonstrate the ability to return to some previously defined shape or size when subjected to appropriate thermal procedure. These materials can be plastically deformed at relatively low temperature, and upon exposure to higher temperature will return to their shape prior to deformation. The basic physics involved in the shape memory effect is a reversible martensitic transformation. Repeated heating and cooling of these alloys results in cyclic motion, which provides utility as thermal actuators. This feature may be used in robotic applications where repeated shape, geometry and positions are required.
Shape memory alloy actuators exist in a variety of forms such as ribbons, wires and thin films. The recovery force generated by a constrained shape memory element acts in the direction of the recoverable shape change and this can be used to perform work Bias spring actuators often use SMA wires that contract when heated above their transformation temperatures. During cooling, the wire relaxes and may be elongated, for example using a biasing spring.
One important application of SMA is in force actuators, wherein the SMA component is designed to exert force over a considerable range of motion, often for many cycles. Shape memory alloys have received increasing attention in recent years, especially in the development of innovative engineering systems such as micro-actuators. The use of SMA as actuators in robotic application attempts to take advantage of their large capacities in motion and force transmission. The simplicity of the actuation principle and the compatibility with micro system technologies make these materials very suitable for highly miniaturized and micro-electromechanical systems due to their simplicity of mechanism, high power/weight (or power/volume) ratios, and their clean, noiseless, spark-free, and frictionless operation.
During the lifespan of the SMA actuator, loss of actuation can occur through repeated cycling due to development of plastic strain. The characteristics associated with the transformation vary through cycle deformation, and fatigue occurs under deformation with high cycles. The important external parameters that affect the reliability of SMA actuators are time, temperature, stress, transformation strain, and the amount of transformation cycles. The important internal parameters that determine the physical and mechanical properties are the alloy composition, type of transformation and the lattice structure including defects. Any alteration in SMA actuation properties will necessarily affect the device into which it is incorporated, which may be critical in devices such as micro-actuators which are required to reliably function in a repeatable manner.
Thus, the behavior of the working characteristics when subjected to thermomechanical cycles becomes crucial in designing the SMA element. It is known that the behavior of the SMA material is a function of transformation temperature, stress, and strain. Based on a specific application, it is of great importance to measure these parameters in an SMA actuator of a particular configuration, to reliably predict the lifespan and reliability of the device incorporating it. It is also important to be able to reliably assess the thermomechanical properties of particular shape memory alloys having particular dimensions, to allow the design of SMA actuators with predictable mechanical properties.